Various movement or position transducing systems are currently available. Most of such transducers are able to sense linear, rotary or angular movement. Optical transducers usually consist of a scanning unit and a glass scale having a grating applied thereto. The scanning unit generally includes a light source, a condenser lens for collimating the light, a scanning reticle with index gratings, and a photodetector. The scale is moved relative to the scanning unit and the lines of the scale coincide alternatively with lines or spaces in the index grating producing periodic light fluctuations. The periodic fluctuations of light intensity are converted by the photodetector into electrical signals that are processed to determine position. Optical transducers can provide position measurements with very high accuracy, particularly if laser light sources are used.
Currently, to improve efficiency, most manufacturers prefer using hand-held encoders and other measurement tools on the workshop floor, rather than in contaminant-free inspection rooms. Optical transducers, however, are sensitive to contamination, and are therefore impractical for use in most manufacturing or shop environments. Therefore, expensive and sometimes unreliable environmental seals or other methods of encapsulating the optical transducer are required to keep dust and oils from reaching the transducer. Furthermore, the light source often requires a fairly large current, thus making optical transducers unsuitable for use in battery-powered measuring tools (e.g., hand-held encoders).
Capacitive transducers draw very little current and are therefore well suited for use in battery-powered measurement tools. Capacitive transducers operate under a parallel plate capacitor model. Transmitter and receiver plates are positioned in a stationary member, and are coupled to appropriate voltage generating and read circuitry, respectively. These plates form one plate in the parallel plate capacitor model. The other plate is supplied by a moveable member or scale that includes many spaced-apart plates. As the scale is moved relative to the stationary member, the transmitter and receiver plates capacitively intercouple through each plate in the scale that passes therebetween. The read circuitry detects changes in voltage in the receiver plate as the plates in the scale are moved.
Capacitive transducers, however, require a small gap between the plates in the stationary member and the plates in the scale. The small gap requires tight tolerances, resulting in increased manufacturing costs. Additionally, capacitive transducers are sensitive to contamination, particularly dielectric fluids such as oils. Therefore, as with optical transducers, expensive and unreliable seals are required.
Magnetic transducers are insensitive to contaminations caused by oil, water and other fluids. Magnetic transducers (e.g., Sony Magnescale encoders.TM.) employ a read head that detects magnetic fields and a ferromagnetic scale that is selectively magnetized with periodic, magnetic patterns. The read head senses magnetic field changes in the magnetic scale pattern as the scale is moved, and thereby determines position. Magnetic transducers are affected by small particles, particularly ferromagnetic particles, which are attracted to the magnetized scale. Consequently, magnetic transducers, as with capacitive and optical transducers, must be sealed, encapsulated or otherwise protected to prohibit dust from diminishing their effectiveness.
Inductive transducers are insensitive to cutting oil, water or other fluids and are insensitive to dust, ferromagnetic particles, and so forth. Inductive transducers (e.g., INDUCTOSYN.RTM. type transducers) employ multiple windings on one member, such as a series of parallel hairpin turns repeated on a printed circuit board, which transmit a varying magnetic field that is received by similar windings on another member. An alternating current flowing in the windings of the first member generates the varying magnetic field. The signal received by the second member varies periodically with the relative position between the two members, and therefore the relative position therebetween can be determined by appropriate circuitry. Both members, however, are active, and therefore must be electrically coupled to their respective circuitry. Electrically coupling both members increases manufacturing and installation costs. Furthermore, in the case of rotary encoders, the moving member must be connected via slip rings which increase the cost and decrease the reliability of the encoder.
Several patents of which the inventors are aware attempt to provide a motion or position transducer that is insensitive to contaminants, yet which can be more inexpensively manufactured than optical, capacitive, magnetic or inductive transducers. U.S. Pat. Nos. 4,697,144, to Howbrook, 5,233,294, to Dreoni, and 4,743,786, to Ichikawa et al., and British Patent Application No. 2,064,125, to Thatcher, show position detection devices that sense position between an inactive or unenergized member and an energized member (e.g., between a permanent magnet and a read circuit). While the transducing systems described in these patents eliminate electrical intercoupling between the two moving members (a drawback of inductive transducers), these systems generally fail to provide the high accuracy of prior transducers (e.g., optical or inductive encoders). Additionally, the inactive member in these transducing systems is preferably ferromagnetic so as to produce a strong magnetic field, or is moved within a defined and concentrated magnetic field generated by the active member. Also, the transducing systems under these patents produce discontinuous output signals, which causes inaccurate readings over extended positions or distances. Furthermore, the transducing systems under these patents are generally inapplicable for a wide variety of applications, such as in hand-held measurement tools or as linear, rotary, angular and other types of position transducing applications.
Overall, the inventors are unaware of a motion or position transducer system that is (1) insensitive to contaminants such as oil and ferromagnetic particles, (2) suitable for a wide variety of applications, (3) accurate, and (4) relatively inexpensive to manufacture as compared to prior transducers. A transducing system providing at least these four benefits has until now been unavailable.